Plasma formation and particle acceleration by pulsed laser



Dec. 26, 1967 v. VAL] ET AL 3,360,733

PLASMA FORMATION AND PARTICLE ACCELERATION BY PULSED LASER Filed Nov. 12, 1964 I NVENTORS'. I/lCTO/Z A VAL I I I 1 ,4 rrae/ke Y United States Patent l 3,360,733 PLASMA FORMATION AND PARTECLE ACCELER- ATIGN BY PULSED LASER Victor Vali, Seattle, Wash, and Walter Vali, State Line, N ev., assignors to The Boeing Company, Seattle, Wash, a corporation of Delaware Filed Nov. 12, 1964, 8er. No. 410,465 1 Claim. (Cl. 328-233) The present invention relates to particle accelerators and more particularly to an improved method and means for accelerating particles to high energies by making use of compact and simplified apparatus.

The many and varied uses for devices capable of accelerating particles such as electrons to a high energy state are well known at the present time. Various techniques and devices have been used for obtaining extremely high energy charged particles with several such devices being capable of producing particles having energies higher than electron volts (referred to as bev.). One device for accelerating charged particles is the Van de Graaif generator which employs a conveyor belt and spray points for applying charged particles to the belt. The Ibelt then serves to convey the charged particles to an insulated electrode, generally in the shape of a sphere, where the charges are accumulated until a very high potential exists. Various other types of devices have been developed for accelerating particles, including: the cyclotron which accelerates charged particles to high energies by means of an alternating electric field between two electrodes in a constant magnetic field; the cosmotron (or proton-synchrotron) which is in effect a synchrotron modified to provide acceleration of protrons by means of frequency modulation of a radio frequency accelerating voltage and in which the particles move in a circular orbit in an increasing magnetic field with the alternating electric field being in synchronism with the orbital motion of the particles; and the well-known linear particle accelerators.

While the above and other types of accelerators will provide high energy particles, it is noted that such accelerators have the common characteristic of being rela tively large, heavy, and require either an extensive strait path for permitting the particles to travel substantial distances or a heavy magnet assembly is needed to confine the particles to circular orbits. The requirement for such long paths or heavy magnet assemblies together with the need for various other pieces of complex equipment has given rise to costly installations for obtaining particles in the bev. and higher energy range. Also such devices are not readily movable but in general require a permanent installation. In many applications mobility of a high energy particle accelerator is a desirable feature, as for example in the case of mobile X-ray units and accelerators used for laboratory research. Various other advantages and uses for a compact and low cost high energy accelerator will be immediately obvious to those skilled in the art.

It is therefore an object of the present invention to provide an improved method for accelerating particles.

It is another object of the present invention to provide a high energy particle accelerator which is compact in size by comparison to prior art particle accelerators producing particles having energies comparable to those provided by the present apparatus.

A further object of the present invention is to provide a novel method and means for accelerating charged particles such as electrons to extremely high energies by making use of relatively low cost and compact apparatus.

An additional object of the present invention is to provide an improved method for accelerating particles to energies greater than one billion electron volts by making use of relatively compact and lightweight devices.

3,369,733 Patented Dec. 26, 1967 Another object of the present invention is to provide a novel particle accelerator capable of accelerating charged particles to extremely high energies and wherein the apparatus is lightweight and low in cost by comparison to prior art devices which heretofore have been required for producing high energy particles.

In accordance with the teachings of the present invention the beam from a laser (an optical maser) is focused on a cloud of charged particles which prefenably is in the form of a plasma (either metallic or gaseous) or for example which is formed by an accumulation of electrons. The laser can be one of the type well known in the art, as for example the well-known ruby laser, and is preferably of the type which is capable of providing energy of greater than a few tens of gigawatts. Ruby lasers having this energy capability are well known and available at the present time, but in accordance with the concepts of the present invention the energy output of the selected laser is chosen according to the desired particle energy. The cloud of charged particles in the form of a plasma or an accumulation of electrons is established with a density such that electromagnetic radiation in the visible or nearinfrared region will be reflected. One method to be utilized in accordance with the invention for obtaining the cloud of particles is to focus the beam of a high energy laser on a small area of a thin metallic film or sheet of metal foil in a manner such that the laser beam serves to first rapidly vaporize the metal and thereby generate a high density plasma. The laser beam is then focused on the resulting high density plasma and according to the laws of conservation of energy an interchange of momentum occurs between the charged particles and the laser beam photons. The result is that the energy carried by the photons in the laser beam is transferred to the charged particles giving rise to high energy particles. The largest momentum transfer between the laser beam and the charged particles takes place when the beam is totally reflected from the charged particles. It should be noted, however, that the difference in momentum transfer between reflection and absorption is only a factor of two.

In order to reflect most of the laser beam radiation the frequency of the plasma of charged particles is selected to be higher than the [frequency of the laser beam energy. Using a ruby laser and a high density metallic plasma, energies of 10 electron volts and higher can be obtained when the laser has an output power capability in the order of sevenal gigaw atts.

In another embodiment of the invention a cloud of electrons having a high density is formed by a special electron accelerator commonly referred to as a plasma electron beam source. The plasma beam source makes use of a cylindrical or spherical cavity in which an ionized plasma is formed with a small aperture being provided for exit of high energy electrons. In view of the electron density required in the cloud the electron source is preferably operated in a pulsed manner at a very high current. The laser beam is directed through an appropriate opening in the wall defining the the cavity and is focused on the high density cloud of electrons therein. The resulting interaction and transfer of momentum between the laser beam and the electrons causes the electrons to achieve energies greater than one bev. depending upon the laser energy per unit area. Thus a low cost source of very high energy particles is provided Without the need for the heavy and bulky assemblies associated with prior art high energy accelerators. The resulting high energy particles may be directed to hit a suitable target for the production of very high energy X-rays.

As described in detail hereinafter the application of power to the laser can be so controlled that the radiation pressure will be suflicient to overcome the charge separation in a metallic plasma and hence cause acceleration of the freed electrons. If the laser power is supplied at an appropriate slower rate ions as well as electrons will be accelerated. Thus the method and apparatus makes possible the acceleration of electrons or of a plasma.

The above as well as additional advantages and objects of the present invention will be more clearly understood from the following description when read with reference to the accompanying drawing wherein,

FIGURE 1 is a side view in diagrammatic form (and including a vacuum chamber in cross section) of one embodiment of the present invention for producing a high density cloud of particles from a piece of metal foil and for then accelerating the resulting particles to high energies;

FIGURE 1A is an enlarged isometric view of the metal film and support assembly shown in FIGURE 1;

FIGURE 1B is an enlarged isometric view of approximately one-half of one of the square areas of the thin metal film of FIGURE 1A having the laser beam focused thereon and illustrating the initial formation of the plasma;

FIGURE 1C is an enlarged isometric view of a part of the thin metal film and corresponding in general to FIG- URE 1B but showing the cloud of particles as having been formed by the laser beam and being subjected to continued radiation by the beam;

FIGURE 2 is a side view in diagrammatic form (with the vacuum chamber in cross section) of an embodiment of the invention which includes an external source of particles arranged to provide a properly located high density cloud of particles in the path of the laser beam; and

FIGURE 3 is a graph of relative laser beam intensity versus time for illustrating the manner in which the laser beam is used to first generate a plasma and then to accelerate all or part of the resulting particles.

Referring now to the drawing and in particular to FIG- URE 1 there is shown for purpose of illustration a conventional laser indicated generally at and adapted to provide an output beam of electromagnetic radiation indicated at 11. The laser 10 may for example be a ruby laser adapted to operate with a power output in the order of l or more gigawatts. The beam 11 passes through window 12 which is transparent to the radiation from the laser 10. The window 12 will be seen to be embedded in the left end of a vacuum chamber 13 which is evacuated by conventional apparatus. The beam 11 is focused by the lens 14 onto a suitable source of particles indicated as the target 15 which, as described in greater detail hereinafter, is preferably in the form of a thin metallic film appropriately supported. It will be seen that the lens 14 is secured to a lens holder 16 which is in turn carried on a threaded shaft 17 supported by appropriate hearings in the end of the chamber walls 13 and having an adjustment knob 18 disposed outside of the chamber 13. In a similar manner the laser target 15 is held by the target holder 20 which is similarly supported for longitudinal adjustment on the threaded shaft 21 having an adjustment knob 22 disposed outside of the evacuated chamber 13. Any desired object 23 which is to be bombarded by the particles is positioned in the path thereof. The stream of high energy particles is indicated by the dashed line 24.

As explained in greater detail hereinafter, the beam of electromagnetic radiation provided by the laser 10 is focused on a small area of target 15 to thereby produce a high energy concentration for creation of the required plasma. The thickness of the metal film is selected in accordance with the penetration depth of the laser beam being used so that there will be a substantially complete transfer of energy from the laser beam to the volume of metal film on which the beam is focused. By thus selecting the thickness of the metal film an extremely rapid vaporization of the metal takes place to thereby give rise to the required high density plasma. Since the thickness of the film may be in the order of a few thousand Angstrom units it is found that the film itself does not possess sufiicient rigidity to permit accurate positioning thereof in the path of the laser. Accordingly, the film is supported by a fine mesh wire screen as illustrated in FIGURE 1A wherein it will be seen that a wire mesh 15A serves to support a thin film of any selected metal indicated generally at 15B. This will also be seen more clearly in FIG- URE 1B which is an illustration of a greatly enlarged section of the target 15 showing the laser beam 11 as being focused thereon and at an instant of time when the laser beam is in the process of vaporizing a small portion of the metal film 15B. The isometric view of the small section of the target metal is shown in FIGURE 1C at a time shortly after the laser beam has vaporized the metal and has caused the high density plasma to be generated. In the illustration of FIGURE 1C the cloud of plasma indicated generally at 25 is still of sufficient density to cause substantially total reflection of the laser beam and hence a maximum interchange of energy from the laser beam to the plasma takes place.

When the plasma is formed it is at a very high temperature and undergoes rapid expansion until the density thereof decreases to a value where the cloud is no longer of suflicient density to cause reflection of the laser beam. However, due to the rapid rise time of the laser pulse as compared to the rate of expansion of the cloud of plasma, a suflicient energy exchange takes place for particles to be accelerated to energies greater than 1 bev. As seen in FIGURE 1 the high energy particles indicated generally at 24 are directed toward a suitable object which may be termed a particle target indicated at 23. The target 23 may for example be a suitable slab of metal for the production of high energy X-rays in response to the bombardment thereof by the high energy particles.

The manner in which the laser beam 11 is able to first generate the high density plasma and to then cause acceleration of the resulting particles will be more clearly understood if the nature of a cloud of charged particles such as a metallic or gaseous plasma is considered. As I discussed hereinafter, the high density cloud of charged particles can also be an accumulation of electrons. The magnitude of the force being produced by the focused laser beam 11 will be evident from a consideration of the force exerted on a surface when a light beam is reflected therefrom. The force F is:

where c is the velocity of light,

13/ T is the power of the laser pulse, and

k is between 1 and 2, depending on the Doppler shift and whether the beam is reflected or absorbed.

Using a ruby laser having pulse power in the order of several gigawatts (1X10 watts) it will be seen that substantial forces are produced. For example using a ruby laser having a pulse power of 2X10 watts it will be seen that the force is approximately 10' dynes. A laser beam can be focused to an area that is limited only by diffraction and is given by 1rd with atmospheres is =10 atm.

described by the following conservation laws (of momentum and energy):

where:

n is the number of photons that interact with the electron cloud,

h is the Planck constant,

1 is the frequency of the incident beam,

11' is the frequency of the reflected beam,

c is the velocity of light,

N is the number of particles in the absorption volume of the focal point of the beam,

v is the velocity of the recoiling electrons, and

For v-c (3) and (-4) can be written as nhu l T c Nm c( The greatest transfer of momentum from the laser beam to the cloud of charged particles takes place when the beam is totally reflected (the difference in momentum transfer between reflection and absorption being a factor of two). In order to reflect most of the radiation, the plasma frequency of the collection of charged particles mutt be higher than the frequency of the laser beam. For a ruby laser, the frequency of the emitted radiation is 435- secf The plasma frequency is given by 1/ ,:8.9-1O /Z 7 where n is the electron density. Equating these two frequencies gives n =2.4- 10 electrons/cm.

ppmin and:

T is the temperature in kev.,

A is the atomic weight,

n is the electron density,

Z is the ionic charge number,

v is the thermal velocity of electrons, 1/ is the plasma frequency, and

p is the minimum impact parameter.

For example, for metallic electron densities with T=1 kev., t l0 sec. Since the rise time of power in the giant laser pulse is longer than 10 sec., the electrons and ions in the plasma are initially in thermal equilibrium. Using the metal foil B as the laser target, its thickness is chosen such that when the laser pulse power reaches a maximum the plasma has an electron density greater than 10 cm.- and therefore is still reflecting. The lower limit for the thickness of the metal film is set by the penetration depth of a laser beam in a metallic plasma, approximately 3000 A. for n =l0 There are about 10 free electrons in a cubic micron of metal which is the approximate volume of the metal vaporized by the laser beam in FIGURE 1. To separate these electrons from the same number of ions, the maximum force necessary is:

N e (1O -4.8-1O T where N is the total number of electrons to be separated from ions, r is the radius of the plasma cloud, and e is the electron charge. This is to be compared with the radiation pressure available from the laser beam. When the radiation pressure is sufiicient to overcome the charge separation, only the electrons are accelerated. If the laser energy is applied more slowly l0 watts/cm?) the ions will be dragged along with the electrons. Therefore, depending on the power of the laser beam and the total number of electrons, either an electron accelerator or a plasma accelerator is provided. From the above equations it will be seen that with a laser power of a few tens of gigawatts particles with energies considerably greater than 10 ev. are produced.

As set forth above, a cloud of electrons of density 10 cmf will reflect a ruby laser beam. To produce a sphere of electrons of radius 10- cm., with a density n =10 cm.' they must be accelerated to energies of per electron, and focus them to an area of 10 cm. To have at any time during the laser pulse the required electron density, the current has to be larger than 10 amps. This necessitates a pulsed operation of the electron beam.

Referring now to FIGURE 2 it will be seen that the apparatus for accelerating electrons produced by an independent source makes use of a well known and compact laser apparatus 10 which includes a laser rod 30 which may be a ruby rod. A pair of optical pumps 31 and 32, such as men flash lamps, are positioned to selectively irradiate the laser rod 30. A selectively actuated shutter assembly or Q-switch 33 is positioned between the left end of the rod 10 and a first reflector 34 having a completely reflective surface. A second reflector 35 adjacent the right end of the rod 10 is only partially reflective and is adapted to permit the passage of the desired laser beam 11 therethrough. As is well known in the art, the pumps 31 and 32 serve to provide sufiicient energy to the optical cavity provided by the laser rod 10 to cause atoms in the rod to become excited to a metastable condition. When such atoms return to their stable condition photons are released which travel down the rod and initiate the now well-known laser action. By maintaining the Q-switch 33 in a closed condition until there is a substantial population inversion between the number of atoms in the stable and metastable conditions, the usual series of short duration and relatively low intensity bursts of radiation are suppressed. Then when the Q switch is opened to permit the reflection of energy by the reflector 34 a major burst of energy is obtained. The relative intensity of a typical high intensity output pulse versus time is illustrated in FIGURE 3.

In the apparatus of FIGURE 2 it will be seen that the lens 14 inside the vacuum chamber 13 serves to focus the beam 11 through a first opening 37A in the spherical plasma chamber 37. A suitable particle source 38, which may be a high energy electron source, is coupled with plasma chamber 37 by the evacuated tunnel 39 and serves to provide a high density electron cloud 40 inside the chamber 37. The particle source and chamber 37 can be one of a number available at the present time capable of providing the above indicated required electron density in the path of the laser beam 11. As set forth above, the laser beam interacts with the electrons with a transfer of momentum taking place and a stream of very high energy electrons 41 emerging through opening 37B. A suitable metal target 43 secured in the end wall of the vacuum chamber is bombarded by the high energy electrons causing high energy X-rays 44 to :224-10 dyncs=22 kg.

be generated. For purpose of illustration the X-r-ays are shown as being directed to a suitable photographic plate 4 5 after passing through an object 46 which is to be X-rayed. As is well known in the art the resulting X-rays are of very short wavelength and thus of high energy since the electrons giving rise to the X-rays are of very high energy.

Since the above described apparatus, including the presently available high energy lasers, is compact and low in cost by comparison to previous particle accelerators capable of producing particles having the energies above indicated, it will be seen that an improved method and means for accelerating particles to high energies has been provided. The in-principle upper limit of acceleration of many devices is established by the bremsstrahlung losses, which in the case of linear acceleration becomes significant only when the particle gains energy equivalent to the rest energy within the classical electron radius. Thus for all practical purposes the bremsstrahlung limitation on acceleration of particles is removed in the case of linear acceleration of particles. Accordingly the present invention makes use of a laser to provide acceleration of particles to energies heretofore unattainable.

While the invention has been described by reference to specific apparatus and methods for purpose of illustration, 2"

it is to be understood that those modifications obvious to a person skilled in the art from the teachings hereof are considered to be encompassed by the following claim.

What is claimed is: The method of accelerating particles comprising the steps of focusing the output beam of a pulsed laser on a thin metal film having an initial thickness substantially equal to the penetration depth of the laser radiation, the energy of said pulse being sufiicient to vaporize the irradiated portion of said film during its rise time, thereby forming a plasma sufficiently dense to reflect said radiation, and maintaining the laser beam focused on said plasma during the persistence of said reflective density to cause an interchange of momentum immediately following formation of said plasma.

References Cited UNITED STATES PATENTS 3,015,013 12/1961 Laszlo 313231 X 3,093,767 6/1963 Latferty 313-186 X 3,206,925 9/1965 Schitf 313-63 X 3,265,855 8/1966 Norton 219-421 3,294,970 12/1966 Jenckel 313-63 X OTHER REFERENCES Anderson: Conduction Via Tracks Ionized by a Laser, IBM Technical Disclosure Bulletin, vol. 5, No. 12, May 1963, pages 71 and 72 cited.

Semiconductor Products and Solid State Technology, August 1966, vol. 9, #8, page 62 cited.

ROBERT SEGAL, Primary Examiner.

JAMES W. LAWRENCE, Examiner. 

